Better handle writability changes in triggerWriteOperations (apple#1624)

Motivation:

If a write is buffered in the `PendingWritesManager` and the high
watermark is breached then a writability change will be fired as a
result. The corresponding event when the channel becomes writable again
comes when enough pending bytes have been written to the socket (i.e. as
a result of a `flush()`) and we fall below the low watermark.

However, if a promise associated with a write has a callback
registered on its future which writes enough data to breach the high
watermark (and also flushes) then we will re-entrantly enter
`flushNow()`. This is okay, `flushNow()` protects against re-entrancy
and any re-entrantly enqueud flushed writes will be picked up in the
write-spin-loop.

The issue here is that the `writeSpinLoop` does not monitor for changes
in writability. Instead if checks the writability before it begins and
after it completes, only signalling if there was a change. This is
problematic: the re-entrant write-and-flush could have caused the
channel to become unwritable yet no event will be fired to make it
writable again!

Modifications:

- Store a local writability state in the pending writes manager and
  modify it when we emit writability changes

Result:

- Better protection against re-entrant writability changes.

Linux: make `Epoll` available on Android (apple#1621)

Android is sufficiently Linux-esque that it should be able to use `Epoll`.

Avoid including non-modular headers. (apple#1605)

Motivation:

We included netinet/ip.h in CNIODarwin, which unfortunately is not
modularised. This causes issues when using SwiftPM to create xcodeproj
files. There's no real reason to do it this way, when we can just as
easily abstract the issue.

Modifications:

- Moved netinet/ip.h to the .c file instead of the .h.
- Defined some exported namespaced integers to correspond to the macros.
- Performed platform abstraction in Posix instead of everywhere.

Result:

- Creating xcodeproj files should be fine.

Don't use deadbeef for dead pointers. (apple#1604)

Motivation:

It turns out using this overflows Int on "Any iOS Device"

Modifications:

Switch the first d to a 7.

Result:

Code will compile on "Any iOS Device".

Don't install Jazzy on Xenial (apple#1597)

Motivation:

Ruby on Xenial is too old for the cocoapods downloader which jazzy uses.

Modifications:

Change Dockerfile to not install Jazzy on xenial.

Result:

Dockerfile will build again, but bionic or later is required for docs.

Make UDP connection allocation test more like best practice code. (ap…

…ple#1583)

Motivation:

There is at least a theoretical race to flush before close in prior version.
Having terrible code in the NIO repo is asking for someone to copy it.

Modifications:

flatMap the various parts of the client together which also ensures the
flush is complete before close is called.

Result:

Slightly nicer code, slightly fewer allocations.

ClientBootstrap: allow binding sockets (apple#1490)

Allocation diffing script no longer drops data. (apple#1513)

Allocation diffing script no longer drops data.

Motivation:

Looking at allocations is hard enough without the script getting the allocation counts wrong.

Modifications:

Where multiple allocations resolved to the same "key" only the last allocation was accounted for. I have changed the script to store all allocations - still keyed in the same way. When diffing the total number of allocations is used - if they are different the total and all contributing stack traces are output.

Added a total for all allocations everywhere at the end together with a difference number.

Output all stack traces before and after contributing to a detected diff.

Update the documentation to reflect changes.

Change the threshold for reporting diffs from > 1000 to >= 1000. This means if allocations go from 1000 to 2000 they are reported as a difference rather than a new allocation.

Result:

It is now easier - if somewhat more verbose to compare allocations.

Add 5.3 CI (apple#1498)

(cherry picked from commit 8da802c)

bootstraps: offer ELG validation (apple#1464)

Motivation:

Today, we just expect the ELGs passed to the bootstraps to be the
correct ones, if not, we crash.

Modifications:

Offer an alternative `validatingGroup:` `init` that just returns `nil`
if the ELGs are of the wrong types.

Result:

Easier to work with multi-stack systems for example when the user might
pass an ELG for either NIO on Sockets or NIO on Network.framework.